Reverse sensors give an audible chime when your vehicle is moving close to an object, the chimes (or beeps) get faster the closer you are to an object eventually giving a continuous tone.
Mongoose MPF-S Automatic Front Parking Sensor SystemCall for price
Mongoose MPRS-S Automatic 25mm Reverse Parking SensorsCall for price
Parkmate PTS400M7-F Front Parking SensorsCall for price
Parkmate PTS800MM (Mercedes Benz Style) Front & Rear Parking system with Ultra-Slim LED DisplayCall for price